1. Field of the Invention
The present invention relates to a method for the production of one-material optical fibers which consist of a light guiding core supported by at least one support component within a protective casing.
2. Prior Art
Particularly suitable light conducting fibers having a low attenuation are single-material or one-material optical fibers which are also known are slab coupled dielectric waveguides or rib waveguides. The one-material fiber preferably consists of an amorphous quartz, which can be produced in a very pure form and thus have a low optical attenuation.
Light conducting fibers, which are of the one- material type, are usually produced by assembling a blank having a core supported by a support component within an outer tube, which is of the same material as the core and support component, and the dimensions of the core and tube are selected so that the core is substantially surrounded by air while within the sleeve. The blank is heated and drawn with the outer tube providing a protective sleeve that protects the fiber core and eases its manipulation. An example of the process of forming single-material or one-material optical fibers is discussed in an article by P. Kaiser and H. W. Astle, "Low-Loss Single-Material Fibers Made From Pure Fused Silica", The Bell System Technical Journal, Vol. 53, No. 6, July-August 1974, pp. 1021-1039.
In monomode fibers of this type of construction, the numerical aperture A.sub.N is governed by the following equation EQU A.sub.N = .lambda..sub.O /2 d.sub.s
where .lambda..sub.O is the wavelength of the conducted light and d.sub.s is the thickness of the support component on which the light conducting core is suspended in the protective sleeve. Under unfavorable conditions, for example when a spatially incoherent light source, which may be a luminescence diode, is used, the light power, which is input-coupled into the fiber, increases with the square of the exploitable numerical aperture and thus in the manner of 1/d.sub.s.sup.2 . If it is desired for example to obtain a value of the numerical aperture A.sub.N = 0.5, it is necessary to select d.sub.s .lambda..sub.O. Thus, the thickness of the support component is equal to the light wavelength, which is for instance 0.65 .mu.m.
In the production of one-material fibers or single-material fibers of this type, one usually commences from a blank which is stretched in length at approximately 10.sup.4 times and the cross-sectional area will be decreased by this same factor. The linear reduction in the cross-sectional dimensions is then .sqroot.10.sup.4 = a hundredfold. In practice, surface tensions lead to an adulteration of the ideal similarity transformation. In particular, the surface tension result in a lateral contraction with an undesirable thickening of the support component or components. In order to obtain the thickness of 0.65 .mu.m for the support components in the final fiber, and assuming a deformation by a drawing factor of 10.sup.4, the support components in the blank prior to drawing would need be thinner than 65 .mu.m. However, it is not readily possible to produce and to use such a thin support component which will have a width of a few mm and a length of approximately 100 mm.